The James Webb Space Telescope – Hubble’s mighty successor.

The James Webb Space Telescope (JWST) is regarded as the next generation in space telescopes and is NASA’s flagship to replace the Hubble Space Telescope.

The JWST will give access to a previously unseen universe: from the formation of stars and planets, to the birth of the first galaxies in the early universe. It will be the most powerful and complex telescope ever built and launched into space.

Image Credit: NASA/Northrop Grumman

The James Webb Space Telescope : Artists Impression

What Will Be Its Capabilities?

Look Further Back In Time

While the Hubble Telescope looks at the universe at the visible and ultraviolet range of the spectrum, the James Webb Space Telescope will be different. It will look at the universe primarily in infra-red.

Observing extremely distant objects at the infra-red end of the spectrum is preferable partly because the universe is expanding. Because of this expansion, the light we see from distant objects in the universe is also stretched into longer wavelengths, away from the visible range (what is visible to our eyes), and into the infra-red end of the spectrum.

Although the JWST won’t be the first infra-red telescope in space, it will be able to provide enhanced detail and increased sensitivity like no other. As a result, it will be capable of providing images of galaxies from the early age of the universe.

The JSWT will provide these images partly due to its design as an infra-red telescope and partly due to its huge mirror. It has a much larger mirror than Hubble and this larger light collecting area enables it to see more distant objects and so peer further back in time.

Show Greater Detail Of Newly Forming Stars

Infra-red observations are also an important way of discovering newly forming stars. These stars are surrounded by gas and dust that absorb visible light and so are very difficult or impossible to see.

However infra-red light is less hindered by small dust particles and can peer through the dust and gas of these regions, revealing the processes that lead to star and planet formation.

Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA), and J. Hester

These images compare two views of the star-forming nebula NGC 2174 in the constellation of Orion. Stars and distant galaxies, which are hidden by dust and gas in the visible light image on the left, are revealed in much more detail in the infra-red image on the right.

Discover Exoplanets

An exoplanet is one which orbits a star outside of our solar system, and while we know of thousands of exoplanets, due to technological limitations very few have actually been photographed.

Even so, the James Webb Space Telescope will be able to observe at wavelengths which exoplanets have never been seen before. It will give new insights into these far-flung worlds and in particular some that are potentially earth-like planets.

Image credit: NASA/Ames/SETI Institute/JPL-Caltech

An artist’s impression of the planet Kepler-186f, the first validated Earth-size planet to orbit a distant star in the ‘habitable zone’ — a range of distance from a star where liquid water might pool on the planet’s surface.

How exoplanets are discovered.

These extremely distant worlds can found by an event called a ‘transit’. This is where a planet passes in front of the host star and its presence is detected by a slight dimming of the stars light as the planet passes in front.

An exoplanets existence can also be inferred by a minute wobble from the star caused by the planets gravity as it orbits past. Using this particular method, in collaboration with ground based telescopes, it can help to determine the mass of a planet.

The third method to detect the existence of an exoplanet is by a stellar coronagraph, a relatively new class of instrument similar to a solar coronagraph, and this is where the JWST comes into its own.

A solar coronagraph is a telescopic attachment which uses a disc to block the glare of the Sun in much the same way that during a solar eclipse, the Moon blocks the light of the Sun.

In other words a solar coronagraph produces an artificial solar eclipse that allows us to view atmospheric features of the Sun, that would normally be overwhelmed by the Sun’s glare.

Image credit: ESA&NASA;/SOHO

(The coronagraph image above shows a coronal mass ejection from the Sun. This event blasts around a billion tons of particles into space travelling at millions of miles an hour.)

A stellar coronagraph has to be much more technologically advanced and sensitive to discover exoplanets in this way, and this has proven to be very difficult. Most of the planets detected so far are in the region of 10 thousand to 1 million times fainter than their host star, and this has stretched present day capabilities to the limit.

All that will change with the superb sensitivity and resolution of the JWST. Its on board coronagraphs will allow scientists to view exoplanets that are more like 10 to 100 million times fainter.

The JWST will be uniquely primed to tell us more about the atmosphere of exoplanets. It will be able to determine the planets temperature, mass, colour, weather and vegetation and perhaps even find the building blocks of life.

Greater Understanding Of Our Solar System

As well as studying planets outside our solar system, the James Webb Space Telescopes remarkable sensitivity and resolution will help us build a broader, fuller picture of the objects in our solar system.

Planetary observations will include Mars and the gas giants Saturn and Jupiter. It will observe Mars to help us understand the trace organics in Mars’ atmosphere, and the giant planets to give us a better picture of their seasonal weather.
The JWST will also observe moons and other small bodies such as asteroids and comets.

A whole host of scientific investigations of our own planetary system will be enabled with the JWST, and this will advance our understanding of the solar system.

What Are The Telescopes Main Features?

Image Credit: NASA/Northrop Grumman


The James Webb Space Telescopes 6.5 metre (21 ft) primary mirror is significantly larger than the Hubble’s 2.4 m (7ft 10in) mirror. Composed of 18 hexagonal segments and made of gold-plated Beryllium, a mirror of this size has never been launched into space before.

A roughly circular overall mirror shape is preferred because of the way the light is focused, and the hexagonal shape of each segment means that they fit together without gaps.

These segments are designed to be foldable, and this allows it to just fit in the 16ft payload bay of the rocket which will carry this technological marvel into space.

The 18 hexagonal segments are made of Beryllium for a number of reasons. Beryllium is a light but very strong metal. It’s good at keeping its shape across a range of temperatures and is not magnetic. Beryllium also happens to be a good conductor of heat.

An incredibly thin layer of gold is applied to the JWST primary mirror, as gold improves the mirrors reflection of infrared light.


Although described as the “secondary” mirror, it is one of the most important pieces of equipment on the telescope. Without the proper positioning of the secondary mirror none of the highly anticipated images and revolutionary science would be achievable.

This small 0.74 metre diameter circular mirror has a critical role to play. Once deployed it will face the much larger gold coated primary mirror, where light from distant objects will be reflected into a focused beam at the secondary mirror.

From there the focused beam is directed back through the centre ‘hole’ in the primary mirror to Webb’s powerful scientific instruments which sit behind the primary mirror.


As the JWST will be primarily observing in infrared light, this is essentially heat energy and so the telescope itself must be kept extremely cold to be able to detect these incredibly distant heat signals.

The scientific instruments of the JWST are so sensitive that they could be swamped by other heat sources which could cause havoc with readings from far off stars and galaxies.

Sources of light and heat which could interfere, range from external sources such as the Sun, the Earth and the Moon, or heat emitted from the observatory itself. To protect the telescope from these heat sources, a multi layered sunshield roughly the size of a tennis court is deployed post-launch.

Image Credit: NASA/Northrop Grumman

Sunshield Full-Size Test Unit

The shield has five extremely thin layers, the core material being Kapton coated with aluminium making it highly reflective. Kapton is a high performance plastic which has a high heat resistance and is very stable over a wide range of temperatures. The aluminium coating on the heatshield is itself treated with silicon for toughness.

Once in orbit, the JWST will be positioned so that the Sun, the Earth and the Moon are always on one side. The sunshield then acts as an umbrella to protect the observatory from these external heat sources. This thermal protection will allow the JWST to passively cool to -220°C (-370°F).

The sun-facing side of the sunshield has a purple-magenta hue because of the silicon treated aluminium coating.


The curiously-named ‘spacecraft bus’ is the infrastructure of the James Webb Space Telescope. Made mostly out of composite graphite material, the structure of the spacecraft bus must support the 6.5 ton space telescope.

The spacecraft bus contains many of the major systems that keep the telescope operating such as the solar power array, the computers and the antenna.

The 20ft solar array is one of 40 deployable structures on the James Webb Space Telescope and it is the first to be deployed after launch. The outspread solar panels convert sunlight into electricity which provides the power needed to operate the telescope’s propulsion and communication subsystems, as well as its scientific instruments.

The computer system directs data both to and from the scientific instruments and to the memory storage unit. It also directs data to the radio system, which can send this data back to Earth and receive commands. The computer also controls the pointing and movement of the spacecraft.


The JWST scientific instruments are contained in an area behind the Primary Mirror called the Integrated Science Instrument Module (ISIM). The ISIM could be described as the heart of the James Webb Space Telescope and it contains a package of four main instruments..

The Near Infrared Camera (NIRCam) is the main imager on the JWST that will detect light from many sources. This will range from the earliest galaxies and stars in the process of formation, to stars in nearby galaxies as well as young stars in the Milky Way. NIRCam is equipped with coronagraphs, which will help determine the characteristics of planets orbiting nearby stars.

Image Credit: NASA/Northrop Grumman

 The Near Infrared Camera (NIRCam)

The Near InfraRed Spectrograph (NIRSpec) is an instrument which is used to disperse light from an object into a spectrum. Analyzing the spectrum of an object can provide a wealth of information about its physical properties, such as chemical composition, mass and temperature.

In order to study the thousands of galaxies planned for observation during its 5 year mission, the NIRSpec is designed to observe 100 objects simultaneously. State-of-the-art technology means that the NIRSpec will be the first spectrograph in space that has this remarkable multi-object capability.

The Mid-Infrared Instrument (MIRI) has both a camera and a spectrograph that sees light in wavelengths that are longer than our eyes see. It has sensitive detectors will allow it to see the infra-red light of distant galaxies, newly forming stars, and faintly visible comets as well as objects in the Kuiper Belt.

MIRI’s camera will provide amazing images, in the way that made Hubble a household name. The spectrograph will also provide new physical details of the distant objects it will observe.

Last but no means least of the four instruments is The Fine Guidance Sensor (FGS). This allows Webb to provide precision pointing information for the telescope to obtain high quality images of particular objects.

Where In Space Will It Be?

The James Webb Space Telescope’s distance from Earth compared to Hubble

Compared to The Hubble Space Telescope which is 354 miles (570km) from Earth, the JWST is much further away from our home planet. It will actually be positioned 1 million miles (or 1.5 million kilometres) from Earth.

It is vital that the telescope is kept extremely cold and protected from heat and light sources. These could range from the Sun, the Earth and Moon, or the satellite itself. As Webb will be observing objects in infra-red light, these heat sources would interfere with extremely faint signals from objects a colossal distance away.

1 million miles from Earth, the JWST will actually orbit the Sun at a position called the second Lagrange point (or L2).

L2 is an area where gravity from the Sun and Earth balance the orbital motion of a satellite. This gives the JWST a fixed position relative to the Sun, Earth and Moon, in effect keeping all three to one side of the satellite. This allows the large sunshield to be at its most effective in cooling the satellite to an amazing -225°C.

When Is The Launch Date?

The JSWT has suffered years of launch delays and billions of dollars of cost overruns. Originally due for launch in October 2018, this was delayed until at least May 2020 as further testing was required on the spacecraft.

The launch was postponed again to March 30th, 2021 based on the recommendations of an Independent Review Board. A further delay has since been announced due to the impact of the COVID-19 pandemic.

The James Webb Space Telescope is now due for launch in November 2021.

The James Webb Space Telescope
Image Credit: NASA/Northrop Grumman

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